Semiconductor package with controlled solder bump wetting and fabrication method therefor

ABSTRACT

A semiconductor package and a method of manufacturing a semiconductor package include a substrate having a plurality of lead fingers. A plurality of stud bumps is attached to the plurality of lead fingers. A die having a plurality solder bumps is provided. The plurality of solder bumps is attached to the plurality of stud bumps to form a plurality of electrical connections and provide controlled collapse of the plurality of solder bumps. An encapsulant encapsulates the die, the electrical connections, and the plurality of lead fingers to expose a lower surface of the plurality of lead fingers. The plurality of stud bumps may include a plurality of clusters of stud bumps.

TECHNICAL FIELD

The present invention relates generally to semiconductors, and moreparticularly to a method and apparatus for manufacturing a semiconductorwith controlled solder bump wetting.

BACKGROUND ART

Semiconductors, or computer chips, have made their way into virtuallyevery electrical product manufactured today. Chips are used not only invery sophisticated industrial and commercial electronic equipment, butalso in many household and consumer items such as televisions, clotheswashers and dryers, radios and telephones. As many of these types ofproducts become smaller but more functional, there is a need to includemore chips in these smaller products. The reduction in size of cellulartelephones is one example of how more and more capabilities find theirway into smaller and smaller electronic products.

As electronic technology has progressed, dies having more powerfulfunctions in smaller semiconductor packages have been developed.Electronic products are increasingly light and compact due to theefficient fabrication of many types of high-density semiconductorpackages. One such package is a flip chip semiconductor package.

In a flip chip semiconductor package, bumps are formed on the bondingpads of a die. Each bump contacts a corresponding contact point on aleadframe, or other substrate, so that the die and the leadframe, orsubstrate, are electrically connected. Compared with conventional wirebonding and tape automated bonding (TAB) methods of joining a chip witha leadframe or substrate, the flip-chip design provides a shorteroverall conductive path and hence better electrical performance in asmaller semiconductor package.

The number of bumps is heated so the number of bumps reflows to form anumber of electrical connections between the die and the leadframe orsubstrate. During the reflow process, as the temperature is raised, thesolder bumps collapse. This therefore forms a metallic compound layerbetween the solder bumps and the contact regions on the leads in aneffort to reinforce the bonding between the solder bumps and the leads.The formation of the metallic compound is called a wetting process.However, due to the wetability of the lead frame, after the solder bumpsare bonded to predetermined positions on the leads of the lead frame,the solder bumps still keep collapsing and extending outwardly to spreadon the leads. This over-collapsing of the solder bumps results incracking of the bonds, which adversely effects the electricalconnection. Furthermore, the over-collapsed solder bumps alsosignificantly decrease the height between the die and the leads. Thereduced height has a detrimental effect on subsequent processes insemiconductor fabrication.

Various other methods of bump attachment and bump collapse control forflip chip on a leadframe or substrate have been in practice. In general,the other methods commonly are focused on pre-treatment of the number oflead fingers on the leadframe by laser, etching, masking, or using otherwettable metals. Some make use of solder either dispensed or printed onthe lead fingers. The pre-treatment of the fingers of the leadframetypically involves higher cost in leadframe manufacture by requiringadditional processes that contribute to increasing the manufacturingcycle time and resulting higher yield losses.

One proposed solution involves forming a solder mask on predeterminedpositions of the leads, wherein the solder mask has at least one openingwith a predetermined size for bonding the solder bumps thereto. Thisproposed solution utilizes the opening size of the solder mask forcontrolling the amount of collapse of the solder bumps. As the size ofthe opening increases, the solder bumps can extend outwardly to agreater extent; that is, the larger the collapse amount, the smaller thevertical height of the solder bumps correspondingly. Therefore, with thecontrol in the collapse degree of the solder bumps, the heightdifference between the semiconductor chip and the leads can bepredetermined, thus eliminating the occurrence of the over-collapsing ofthe solder bumps. However, the formation of the solder mask on the leadframe uses processes such as screen-printing or photolithographicpatterning processes, which are quite complex and expensive.

Another proposed solution involves forming a layer of underfill materialwith or without a flux additive over the entire surface of the leads andpositioning the solder bumps into the layer of underfill material untilthe solder bumps contact the leads. This solution precludes the use ofan underfilling process subsequent to die attach thereby increasing thepossibility of the creation of gaps or voids in the underfill material,which adversely effect the performance and reliability of thesemiconductor.

Another proposed solution uses a solder alloy having a higher meltingpoint in an attempt to control the over-collapsing of the solder bumps.However, such solder bumps generally are more expensive.

Solutions to these problems have been long sought but prior developmentshave not taught or suggested any solutions and, thus, solutions to theseproblems have long eluded those skilled in the art.

DISCLOSURE OF THE INVENTION

The present invention provides semiconductor package and a method ofmanufacturing a semiconductor package including a substrate having aplurality of lead fingers. A plurality of stud bumps is attached to theplurality of lead fingers. A die having a plurality solder bumps isprovided. The plurality of solder bumps is attached to the plurality ofstud bumps to form a plurality of electrical connections and providecontrolled collapse of the plurality of solder bumps. An encapsulantencapsulates the die, the electrical connections, and the lead fingersto expose a lower surface of the plurality of lead fingers. Theplurality of stud bumps may include a plurality of clusters of studbumps.

The present invention provides a semiconductor package having a wettablesurface on the lead fingers that controls solder bump wetting, thusachieving control of solder bump collapse and consistent standoffbetween a die and a substrate in a semiconductor package.

Certain embodiments of the invention have other advantages in additionto or in place of those mentioned above. The advantages will becomeapparent to those skilled in the art from a reading of the followingdetailed description when taken with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a semiconductor package at an intermediatestage of manufacture in accordance with the present invention;

FIG. 2 is a cross-sectional view of the structure of FIG. 1 taken alongline 2—2;

FIG. 3 is the structure of FIG. 2 after formation of a stud bump on alead finger;

FIG. 4 is an enlarged top view of a lead finger of a leadframe having acluster of two stud bumps;

FIG. 5 is an enlarged top view of a lead finger of a lead frame having acluster of three stud bumps;

FIG. 6 is an enlarged top view of a lead finger of a lead frame having acluster of four stud bumps;

FIG. 7 is an enlarged top view of a lead finger of a lead frame having acluster of five stud bumps;

FIG. 8 is a cross sectional view of the semiconductor package before dieattach and encapsulation;

FIG. 9 is a cross-sectional view of the structure of FIG. 8 after dieattach and encapsulation; and

FIG. 10 is a flow chart of a method for manufacturing a semiconductorpackage in accordance with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following description, numerous specific details are given toprovide a thorough understanding of the invention. However, it will beapparent that the invention may be practiced without these specificdetails. In order to avoid obscuring the present invention, somewell-known system configurations and process steps are not disclosed indetail.

Likewise, the drawings showing embodiments of the devices aresemi-diagrammatic and not to scale and, particularly, some of thedimensions are for the clarity of presentation and are shown greatlyexaggerated in the FIGs. Generally, the device can be operated in anyorientation. In addition/Also, where multiple embodiments are disclosedand described having some features in common, for clarity and ease ofillustration and description thereof like features one to another willordinarily be described with like reference numerals.

The term “horizontal” as used herein is defined as a plane parallel tothe conventional plane or surface of the die, regardless of itsorientation. The term “vertical” refers to a direction perpendicular tothe horizontal as just defined. Terms, such as “on”, “above”, “below”,“bottom”, “top”, “side”, “higher”, “lower”, “over”, and “under”, aredefined with respect to the horizontal plane.

The term “processing” as used herein includes deposition of material orphotoresist, patterning, exposure, development, etching, cleaning,and/or removal of the material or photoresist as required in forming adescribed structure.

Referring now to FIG. 1, therein is shown a plan view of a semiconductorpackage 100 at an intermediate stage of manufacture in accordance withthe present invention. The semiconductor package includes a leadframe102, or other suitable substrate. It will be apparent to those skilledin the art upon a reading of this disclosure that the leadframe 102 canbe any substrate having wettable lead fingers or contacts upon which asemiconductor device is mounted using solder balls or bumps. Typically,the substrate includes at least one of a leadframe, a printed wiringboard, a flame-retardant fiberglass (FR4) board, an organic circuitboard, a ceramic substrate, a hybrid circuit substrate, an integratedcircuit package, a semiconductor substrate, a polyimide tape, a flexcircuit, a high-density interconnect board, an electronic module, andcombinations thereof.

The leadframe 102 has an outer frame 104 and a number of lead fingers106. The number of lead fingers 106 extends inwardly from the outerframe 104. The number of lead fingers 106 has a number of solder bumpcontact areas 108. The leadframe 102 typically is made of a conductivematerial, such as copper.

Referring now to FIG. 2, therein is shown a cross-sectional view of thestructure of FIG. 1 taken along line 2—2. The number of lead fingers 200is representative of the number of lead fingers 106 shown in FIG. 1. Thenumber of lead fingers 200 has an upper surface 202. The number of leadfingers 200 is at least one of copper, a substrate material plated withbondable metal, a substrate material plated with a wettable metal, asubstrate material selectively plated with a bondable metal, a substratematerial selectively plates with a wettable metal, and combinationsthereof. The number of lead fingers may be selectively platedsubstantially only in the number of solder bump contact areas 108 withthe foregoing metals.

Referring now to FIG. 3, therein is shown the structure of FIG. 2 afterformation of a number of stud bumps 300 on the upper surface 202 of thenumber of lead fingers 200. The number of stud bumps 300 is formed usinga wire bonding process. The wire bonding process is performed withconventional wirebonding equipment that has stud bumping capability.Typically, the number of stud bumps is at least one of gold (Au), copper(Cu), and combinations thereof.

The number of stud bumps 300 is formed using a wire of a diameter thatprovides the number of stud bumps 300 that is smaller than orsubstantially equal to the size of the solder bumps on the die to beattached to the number of lead fingers 200 as discussed below. Thenumber of stud bumps 300 having, for example, a diameter of 50–60 um,will require a substantially larger diameter wire than is used forstandard wire bonding using conventional wirebonding equipment with studbump capability. The number of stud bumps 300 is coined or sheared toform a flat bondable upper surface.

Referring now to FIG. 4, therein is shown an enlarged top view of a leadfinger 400 of a leadframe (not shown) having a cluster 402 of a numberof stud bumps 404. In FIG. 4, the number of stud bumps 404 forming thecluster 402 is two. The number of stud bumps 404 is formed near eachother to form the cluster 402. The combined diameter of the number ofstud bumps 404 is smaller than or substantially equal to the diameter ofthe solder bumps on the die to be attached to the lead finger 400.Accordingly, a smaller diameter wire is used to form the number of studbumps 404 having a given total diameter than would be required to formone stud bump having a diameter that is smaller than or substantiallyequal to the diameter of the solder bumps.

Referring now to FIG. 5, therein is shown an enlarged top view of a leadfinger 500 of a lead frame (not shown) having a cluster 502 of a numberof stud bumps 504. In FIG. 5, the number of stud bumps 504 forming thecluster 502 is three. The number of stud bumps 504 is formed near eachother to form the cluster 502. The combined diameter of the number ofstud bumps 504 is smaller than or substantially equal to the diameter ofthe solder bumps on the die to be attached to the lead finger 500.Accordingly, a smaller diameter wire is used to form the number of studbumps 504 having a given total diameter than would be required to formone stud bump having a diameter that is smaller than or substantiallyequal to the diameter of the solder bumps.

FIG. 6 is an enlarged top view of a lead finger 600 of a lead frame (notshown) having a cluster 602 of a number of stud bumps 604. In FIG. 6,the number of stud bumps 604 forming the cluster 602 is four. The numberof stud bumps 604 is formed near each other to form the cluster 602. Thecombined diameter of the number of stud bumps 604 is smaller than orsubstantially equal to the diameter of the solder bumps on the die to beattached to the lead finger 600. Accordingly, a smaller diameter wire isused to form the number of stud bumps 604 having a given total diameterthan would be required to form one stud bump having a diameter that issmaller than or substantially equal to the diameter of the solder bumps.

FIG. 7 is an enlarged top view of a lead finger 700 of a lead frame (notshown) having a cluster 702 of a number of stud bumps 704. In FIG. 7,the number of stud bumps 704 forming the cluster 702 is four. The numberof stud bumps 704 is formed near each other to form the cluster 702. Thecombined diameter of the number of stud bumps 704 is smaller than orsubstantially equal to the diameter of the solder bumps on the die to beattached to the lead finger 700. Accordingly, a smaller diameter wire isused to form the number of stud bumps 704 having a given total diameterthan would be required to form one stud bump having a diameter that issmaller than or substantially equal to the diameter of the solder bumps.

It has been discovered that a standard diameter wire, typically in therange of about 0.6–1.2 mils in diameter, may be used to form the clusterof the number of stud bumps resulting in the ability to use conventionalwire diameters to form a variety of diameters of the cluster of studbumps without altering existing wirebonding manufacturing equipment. Thenumber and arrangement of the number of stud bumps can be varied to meetthe requirements of a particular semiconductor package. Additionally, ithas been discovered that use of a single stud bump or a cluster of studbumps enhances the integrity of the solder joint made by connecting thesolder bumps on the die to the single stud bump or a cluster of studbumps on the lead fingers and controls the collapse of the solder bumps.

Referring now to FIG. 8, therein is shown a cross sectional view of thesemiconductor package 100 after prior to die attach and encapsulation. Adie 800 has a number of solder bumps 802 formed on the lower surface ofthe die 800. The number of solder bumps 802 is at least one of a leadedsolder, eutectic, lead-free alloy, and combinations thereof. The numberof solder bumps 802 may be formed, for example, by electroplating one ormore metals such as lead and tin to form a lead-tin solder bump. Thenumber of solder bumps 802 also may be formed by depositing layers ofone or more metals on an interconnection surface of the die and usingconventional photolithographic techniques to pattern and etch anyundesired metal. The number of solder bumps 802 also may be heat treatedto melt the number of solder bumps 802 to form a rounded shape.Alternatively, the number of solder bumps 802 may be formed bypositioning solder balls or bumps on the contact pads (not shown) of thedie 800 and heating the solder balls or bumps to adhere them to thecontact pads of the die 800. Alternatively, the number of solder bumps802 may be formed by selectively screen printing solder paste on thecontact pads on the die 800, and then heating the die 800 to melt thesolder paste and form the number of solder bumps 802.

The die 800 is placed on a number of lead fingers 804 using aconventional die pick and place process. The number of solder bumps 802on the lower surface of the die 800 typically is dipped in a flux toproduce a perimeter of wettable area on the number of solder bumps 802.The flux also may be sprayed, printed, or otherwise applied to thenumber of solder bumps 802.

Referring now to FIG. 9, therein is shown the structure of FIG. 8 afterdie attach and encapsulation. The number of solder bumps 802 is placedinto a number of stud bumps 806. The number of stud bumps 806 shown inFIGS. 8 and 9 is representative of the number of stud bumps 300 shown inFIG. 3, the cluster 402 of the number of stud bumps 404 shown in FIG. 4,the cluster 502 of the number of stud bumps 504 shown in FIG. 5, thecluster 602 of the number of stud bumps 604 shown in FIG. 6, and thecluster 702 of the number of stud bumps 704 shown in FIG. 7.

The number of solder bumps 802 displaces a centrally located portion ofthe number of stud bumps 806. The number of stud bumps 806 surrounds abase portion of the number of solder bumps 802 when the number of studbumps 806 is displaced by the number of solder bumps 802.

The number of solder bumps 802 is reflowed by heating to form aninterconnection between the die 800 and the lead fingers 804. Theassembled die 800 and the number of lead fingers 804 are heated, such asby using at least one of an infrared, convective, forced-air, andcombinations thereof, furnace to heat the number of solder bumps 802 andthe number of stud bumps 806. An electrical connection between thenumber of solder bumps 802 and the lead fingers 804 is formed. Thenumber of solder bumps 802 flows around a portion of the number of thenumber of stud bumps 806 and become soldered to the lead fingers 804.The number of stud bumps 806 control the collapse of the number ofsolder bumps 802. Alternatively, the number of solder bumps 802 may belocally heated and pressed against the lead fingers 804 to displace thenumber of solder bumps 802 around the number of stud bumps 806, and toreflow the number of solder bumps 802.

An encapsulant 900 is formed over the die 800, the number of solderbumps 802, and the lead fingers 804 using a molding compound, such as anepoxy.

Referring now to FIG. 10, therein is shown a flow chart of a method 1000for manufacturing a semiconductor package in accordance with the presentinvention. The method 1000 includes providing a substrate having aplurality of lead fingers in a block 1002; attaching a plurality of studbumps to the plurality of lead fingers in a block 1004; providing a diehaving a plurality solder bumps in a block 1006; attaching the pluralityof solder bumps to the plurality of stud bumps to form a plurality ofelectrical connections and provide controlled collapse of the pluralityof solder bumps in a block 1008; and encapsulating the die, theelectrical connections, and the lead fingers to expose a lower surfaceof the plurality of lead fingers in a block 1010.

Thus, it has been discovered that the method and apparatus of thepresent invention furnish important and heretofore unavailablesolutions, capabilities, and functional advantages for semiconductormanufacturing. The resulting process and configurations arestraightforward, economical, uncomplicated, highly versatile andeffective, use conventional technologies, and are thus readily suitedfor manufacturing semiconductor devices that are fully compatible withconventional manufacturing processes and technologies.

While the invention has been described in conjunction with a specificbest mode, it is to be understood that many alternatives, modifications,and variations will be apparent to those skilled in the art in light ofthe foregoing description. Accordingly, it is intended to embrace allsuch alternatives, modifications, and variations that fall within thescope of the included claims. All matters hithertofore set forth hereinor shown in the accompanying drawings are to be interpreted in anillustrative and non-limiting sense.

1. A method of manufacturing a semiconductor package, comprising:providing a substrate having a plurality of lead fingers; attaching aplurality of stud bumps to the plurality of lead fingers; forming theplurality of stud bumps to form flat bondable surface thereon; providinga die having a plurality solder bumps; attaching the plurality of solderbumps to the plurality of stud bumps to form a plurality of electricalconnections and provide controlled collapse of the plurality of solderbumps; and encapsulating the die, the electrical connections, and thelead fingers to expose a lower surface of the plurality of lead fingers.2. The method of manufacturing a semiconductor package as claimed inclaim 1, wherein: providing the plurality of solder bumps provides aplurality of solder bumps having a first diameter; and attaching theplurality of stud bumps attaches a plurality of stud bumps having asecond diameter smaller than or substantially equal to the firstdiameter.
 3. The method of manufacturing a semiconductor package asclaimed in claim 1, wherein attaching the plurality of stud bumps uses awire bonding process.
 4. The method of manufacturing a semiconductorpackage as claimed in claim 1, wherein: attaching a plurality of studbumps attaches a plurality of clusters of stud bumps; and the pluralityof clusters of stud bumps comprise at least two stud bumps.
 5. Themethod of manufacturing a semiconductor package as claimed in claim 1,wherein attaching the plurality of stud bumps attaches stud bumps of atleast one of copper, gold, and a combination thereof.
 6. A method ofmanufacturing a semiconductor package, comprising: providing a leadframehaving a plurality of lead fingers; attaching a plurality of stud bumpsto the plurality of lead fingers; forming the plurality of stud bumps toform flat bondable surfaces thereon; providing a die having a pluralitysolder bumps; vertically aligning the plurality of solder bumps to theplurality of stud bumps; reflowing the plurality of solder bumps to forma plurality of electrical connections with the plurality of stud bumpsand provide controlled collapse of the plurality of solder bumps; andencapsulating the die, the electrical connections, and the lead fingersto expose a lower surface of the plurality of lead fingers.
 7. Themethod of manufacturing a semiconductor package as claimed in claim 6,wherein: providing the plurality of solder bumps provides a plurality ofsolder bumps having a first diameter; and attaching the plurality ofstud bumps attaches a plurality of stud bumps having a second diametersmaller than or substantially equal to the first diameter.
 8. The methodof manufacturing a semiconductor package as claimed in claim 6, whereinattaching the plurality of stud bumps uses a wire bonding process. 9.The method of manufacturing a semiconductor package as claimed in claim6, wherein: attaching a plurality of stud bumps attaches a plurality ofclusters of stud bumps; and the plurality of clusters of stud bumpscomprise at least two stud bumps.
 10. The method of manufacturing asemiconductor package as claimed in claim 6, wherein attaching theplurality of stud bumps attaches stud bumps of at least one of copper,gold, and a combination thereof.